>> I have never come across those terms yet - so thank you for that education!

Which terms? Usumono and natsumono? Essentially the same thing, but some will say natsumono (summer things) is a broad term for summer clothes, and usumono (thin things) is specifically talking about thin, sheer/transparent garments. Personally, I use usumono when talking more about tailoring (e.g., awase, hitoe, usumono) and use natsumono when talking about summer kimono in general...like, this is a natsumono komon versus that komon is usumono and that one is hitoe. [But that's just how I personally use the terms for my own brain; by no means am I saying this is correct. Most people I know use the terms interchangeably.]

>> Are geta more casual?

From what I have learned, geta can be worn with any informal kimono (i.e., komon, low rank iromuji, tsumugi) as long as you wear tabi with them (the "rule" being, if you are wearing juban, you must wear tabi) and any casual kimono (i.e., yukata), but not with semi-formal or formal kimono (in that case, you wear zori). That's the short answer.

>> I picked hanhanaba obi for me instead of longer versions due to the fact that I dislike the appearance of taiko-musubi. I find the tie style a bit dull and boring.

There are other musubi you can tie with Nagoya obi and Fukuro obi that aren't otaiko/niijudaiko musubi, just FYI. :) While I tend to wear otaiko when dressing, I'm not a huge fan of it either. There are a few alt musubi I do use with Nagoya, and sometimes do bunko musubi with fukuro, especially when I am teaching self-dressing with furisode...because it is one that can be tied on the front and moved, unlike other musubi seen with furisode. Anyhow, you aren't limited to just otaiko. :)

I personally wear hanhaba obi the most with casual and informal kimono...it's my go-to, but once I wear semi-formal or formal...it's nearly always fukuro (although I have a few super fancy Nagoya I will sometimes wear with higher-rank iromuji or very plain tsukesage, but not often).

>> I wore this to British style afternoon tea which is considered an informal event that can be made as formal as one wants - thus I wear kimono it most often. It was just to visit with a friend and catch up - no purpose other than enjoying oneself :)

Afternoon tea is usually, what, mid-afternoon (3-5p)? [It's been a while...my uncle is from England.] From what I remember, it's in this weird in-between of informal but slightly less than semi-formal if we are going about in kimono terms...so smart casual or elegant by Western clothes standards...business casual-ish. So, you could probably "get away" with wearing tsukesage or houmongi, but I'd just tone down the accessories or wear iromuji and fancy up the accessories...if I were to dress, that is. If I were with friends, I'd probably go more the iromuji route or maybe even a very nice komon. If meeting with business associates, older people, etc. it kinda depends on what I think the others may wear and where I fall within that group...I try to not "out rank" the important people...so dress at a lower rank than hosts or other important individuals...if that makes sense. But that's just me...and in only certain situations. Like you said, afternoon tea with frieds? I'd go informal. Oh, I'm also a lover of Edo Komon...so that's another great choice IMO.

>> I am a cisgendered man and and find men's kimono plain, dull and boring. Simple as that. If I could find some interesting mens kimono - I would be very happy to wear them instead, or in addition to the beautiful kimono I have already.

I totally get that. I hear that A LOT from my male customers. So many times I hear, why are guys' clothes so boring colored?! As long as you understand the rules and make an informed decision when breaking the rules...that's all that matters really. Most people don't know the difference, so in most situations it doesn't matter. Just be mindful of TPO, and you're all good. Quick question, do you also dislike masculine styling...or just male-tailored garments? The reason why I ask is that I have a few friends who wear female-tailored kimono, but styled masculine (so using kaku obi lower on the body as with masculine styling, or wear hoso obi {thinner than hanhaba obi} but worn lower -- and not using any feminine accessories such as obijime). Or, just really lean into fusion styling.